A wind turbine generator 1 (hereinafter also referred to as “wind turbine”), for example, as shown in FIGS. 7 to 9, is an apparatus that generates electric power with a generator that is driven in such a manner that a rotor head 4 equipped with wind turbine blades 5 rotates when receiving wind power and increases the rotational speed using a gearbox.
In such a wind turbine generator 1, since the rotor head 4 equipped with the wind turbine blades 5 is joined to a gearbox and a generator in a nacelle 3 installed on the top of a tower 2 via a main shaft, for example, an up-wind wind turbine generator 1 needs to receive the wind from the front of the rotor head 4 by yawing (turning on a substantially horizontal plane) the nacelle 3 on the tower 2 to point the rotor head 4 in a fluctuating wind direction (to make the rotational plane of the rotor face the wind direction).
FIGS. 8A and 8B show a configuration example of a yaw driving unit 10 for yawing the nacelle 3. The yaw driving unit 10 is a yaw control system for yawing the large nacelle 3 using the driving force of a yaw motor 11 so that the rotational plane of the rotor follows the wind direction to face it.
Reference sign 2 in the drawing denotes a tower, 3 denotes a nacelle, 3a denotes a nacelle base plate, 12 denotes a drive gear (pinion gear), 13 denotes a fixed gear, 14 denotes a yaw slewing bearing (rolling bearing), and 15 denotes a yaw braking system.
Specifically, in the configuration example in FIGS. 8A and 8B, the rolling bearing serving as the yaw slewing bearing 14 is configured such that rolling elements 14c are held between an outer ring 14a and an inner ring 14b, and a fixed gear 13 is formed around the outer diameter (outer circumferential surface) of the outer ring 14a, which is fixed to the tower 2. The inner ring 14b is fixed to the nacelle base plate 3a. 
On the other hand, because the drive gear 12 that engages with the fixed gear 13 is rotated by the yaw motor 11, which is securely fixed to the nacelle base plate 3a, the nacelle 3 turns together with the drive gear 12 that orbits around the outer circumference of the fixed gear 13 while rotating on its axis.
The above-described yaw slewing bearing 14 may have a configuration in which the fixed gear 13 is formed around the inner diameter (inner circumferential surface), with the inner ring 14b serving as a fixed side, so that the drive gear 12 orbits around the inner circumference of the fixed gear 13 while rotating on its axis.
The wind turbine blades 5 are each provided with a pitch driving unit 16 to vary the pitch angles of the wind turbine blades 5, as shown in FIG. 9. In the pitch driving units 16, the wind turbine blades 5 are each supported via a blade slewing bearing (rolling bearing) 17 so that they can turn relative to the rotor head 4.
Conventional pitch driving units are provided with some play so that the pinion gear can slightly move in the axial direction to prevent the teeth from being broken due to partial contact between the teeth of the blade slewing gear and the teeth of the pinion gear. (For example, refer to Patent Literature 1)